The present invention relates to vacuum interrupter electrical apparatus and more particularly to the electrical contacts of such apparatus. Vacuum interrupters find application as circuit protection devices in electrical distribution systems, and comprise a sealed envelope with movable contacts disposed within the envelope for making and breaking electrical continuity. When the contacts are in a closed current carrying position in contact with each other, the contact must carry large currents efficiently with low resistance values. When the contacts are moved apart to the open circuit position an arc strikes between the contacts vaporizing some portion of the contact with a rapid quenching of the arc and interruption of the circuit. The contacts must be readily separable, i.e., have an antiweld characteristic so that the operating mechanism need not exert undue force in moving the contacts apart. While some vaporization of the contact material is necessary to sustain the arc, gross erosion of the contacts is to be avoided since this will give rise to high contact resistance when the contacts are closed for current carrying operation.
The selection of contact materials is therefore a very critical aspect in the functioning of the whole vacuum interrupter apparatus. A widely used contact material is a blend of a high-conductivity material such as copper with a higher melting point refractory metal such as chromium or tungsten. A widely used chromium matrix contact material which is infiltrated with copper is seen in U.S. Pat. No. 3,818,163. The contact is formed by sintering the chromium to form a porous hardened body which is then brought into contact with copper which is melted to infiltrate it into the pores of the chromium matrix. In another variation of this matrix contact, seen in U.S. Pat. No. 3,960,554, a small portion of copper powder is blended with the chromium powder prior to sintering of the chromium to form a matrix. This small blend of copper increases the green strength of the chromium compact during sintering which facilitates handling of the compact. The matrix contact is thereafter infiltrated with additional copper to form the completed contact.
A more recent copper-chromium contact material is described in U.S. Pat. No. 4,032,301 in which the copper and chromium are admixed as powders, and cold pressed to an intermediate density. The pressed compact is then vacuum sintered below the copper melting point to densify the contact. The contact is then hot pressed or hot-densified to achieve a final high density. The copper and chromium are present in a weight ratio of about 1 to 1. The copper content is maintained at below about 60 wt.% in order to insure successful contact operation.
It had been well known in the powder metallurgical art to compact powdered metal mixtures and press at high pressures to densify the compact to some extent, and to thereafter sinter at temperatures below melting points of the powdered metals. Such contacts are seen in U.S. Pat. No. 2,362,007 which has up to about 10% chromium, some phosphorous, with the remainder being copper. U.S. Pat. No. 2,758,229 describes a wear-resistant electrical current commutator which is approximately 80% copper, 20% chromium and is formed by compacting the admixed powders at about 14 tons psi, and thereafter sintering at from 800.degree. to 1000.degree. C. The sintered contact is then preferably impregnated with an organic oil to further improve the wear resistance of the commutator contact.
It remains a goal to produce as high a conductivity contact for vacuum interrupter usage as is possible consistent with the need for structural rigidity, anti-weld characteristic, high-voltage withstand capability, and low outgassing from the contact during arcing.